On Thu, 15 Jul 1999, John Steczkowski wrote:
> The problem is that the eccentric shaft (i.e. crankshaft) doesn't turn once
> per rotor revolution, it's geared up, so one full rotation of a rotor is
> more than one rpm. I seem to remember that a 2 rotor rotary gives the same
> number of power pulses as a 6 cylinder.
That's correct, sort of.
>From the 1st gen RX-7 faq:
"It takes 3 revolutions of the eccentric shaft for one revolution of the
rotor (i.e. for, say, the same apex to be in the same place). Hence, each
rotor (of which there are two in Rx-7s) fires once for each revolution of
the eccentric shaft. Compared to a 4-stroke, it fires as often as a
4-cylinder. However, each power stroke lasts for 2/3 of an eccentric shaft
revolution, so it has the same amount of time of the revolution of the
main shaft (crank or eccentric) under power as a 6-cylinder 4-stroke."
If you multiply the displacement by 2, you get a pretty good
approximation, i.e. 1.3L 13B-REW (93+ RX-7) gets 255 hp, which would not
be unreasonable for a twin turbo 2.6L 6 cylinder.
The problem is (and there was just a flame war on this subject over on the
RX-7 list) that it's really apples to oranges, and 2X just happens to work
(fairly) well in the case of the Mazda rotaries. It wouldn't work at all
if someone came along with a 4-faced rotor, for instance.
Considering that a twin-turbo 3-rotor (2.0 liter) can be made to produce
550 HP and 460 lb-ft of torque, it makes sense to keep that out, and the
2x works, as it keeps the 2-rotors within the class and the 3-rotors out.
HOWEVER, someone (probably someone insane) COULD, IN THEORY, build a
three-rotor out of 60mm pre-1974 rotors and housings, and squeak in at 3.0
liters. I say let 'em. But I have no idea how well such a thing would
perform.
Now if only you'd allow 2-seaters ;-)
Steven
'93 RX-7
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